We assume you checked the fuses on the truck, and the emergency flashers also. If the emergency flashers work on the trailer, it is in the vehicle.
You can also cross plug the brown wire from the vehicle, to the green or yellow wire with the parking lights on. This will activate the brake lights in the parking mode. Just a simple check to check the wires.
Since you have no brake lights, we would think it is a grounded out parking light or light bar that may have blown a fuse.
BCB